Exercised: Why Something We Never Evolved to Do Is Healthy and Rewarding

by Daniel E. Lieberman

Muscles benefit from all physical activities, but they respond most strongly to weight-bearing activities that require them to contract forcefully without changing length (isometric contractions) or as they lengthen (eccentric contractions). To prevent sarcopenia, do weights.

According to one estimate, three to four hours of moderate exercise a week is likely to reduce a woman’s risk of breast cancer by 30 to 40 percent, and both men’s and women’s risk of colon cancer by 40 to 50 percent.94

An especially promising discovery is that vigorous exercise potently enhances the effectiveness of natural killer (NK) cells,

elderly Tsimane with the same ApoE4 gene are less likely to show declines in cognitive performance if they suffer from many infections.108 Alzheimer’s may thus be an example of an evolutionary phenomenon called the hygiene hypothesis. According to this idea, ApoE4, which can be expressed by cells in the brain, might have evolved long ago to help protect the brain when infectious diseases were ubiquitous. Those of us today who live in bizarrely sterile environments without many germs and worms, face an increased chance that these formerly protective immune

mechanisms turn against us. (The hygiene hypothesis also helps explain increased rates of allergies and many other autoimmune diseases.109) How Does Physical Activity Help?

An analysis of sixteen prospective studies including more than 160,000 individuals found that moderate levels of physical activity lowered the risk of Alzheimer’s by 45 percent.111 More physically intense activities may be associated with reduced risks for the disease.112 Physical activity also slows the rate of cognitive and physical deterioration in Alzheimer’s patients.

The most well supported is that physical activity—especially of longer duration but also more vigorous activities—causes the brain to produce a powerful molecule known as BDNF (brain-derived neurotrophic factor).

BDNF is a sort of growth tonic for the brain that nourishes and induces new brain cells, especially in regions involved in memory. But because we never evolved to be persistently sedentary, we never evolved a mechanism other than physical activity to produce high levels of BDNF.

In a classic mismatch, absence of exercise deprives us of doses of BDNF that have been shown to improve memory and cognition and to maintain neuronal health that apparently helps prevent Alzheimer’s.

Rodents that run on treadmills develop fewer plaques and tangles in their brains and have lower levels of inflammation associated with Alzheimer’s.

One analysis of nineteen studies found that aerobic physical activity is most beneficial, but other reviews favor a mix of aerobic exercise, weights, and exercises that improve balance and coordination.120 In addition, limited evidence suggests there may be a dose-response relationship between exercise intensity and risk.121

“Mens sana in corpore sano” (a sound mind in a healthy body).

Unlike grief, depression tends to be persistent and characterized by feelings of low self-worth and guilt.

Likewise, it may sometimes be adaptive to be discouraged and unmotivated, hence disinclined to engage in behaviors unlikely to be successful like fighting someone who might kill us or wooing a lover who spurns us. In depression disorder, however, these low moods become directed persistently at ourselves, not the outside world.

armamentarium.

As a reminder, exercise heightens the activity of transmitting molecules in the brain, notably dopamine, serotonin, and norepinephrine.131 These neurotransmitters induce sensations of reward,

Exercise also increases levels of other neurotransmitters, glutamate and GABA, that are often depleted in people with depression and anxiety.132 Additional mood-enhancing molecules turned on by exercise include endogenous opioids such as endorphins and endocannabinoids that inhibit pain and produce positive moods.133 Finally, as if this were not enough, remember that physical activity increases levels of BDNF and other growth factors that help maintain brain function.

For similar reasons, exercise also benefits other neural and cognitive disorders from attention deficit hyperactivity disorder to Parkinson’s. Exercise has additionally been shown to modestly but significantly improve memory, attention span, and various aspects of cognition including math and reading abilities.

That avoidance makes sense because, as we have seen from the very start of this book, exercise is a fundamentally strange and unusual behavior from an evolutionary perspective. Thirteen chapters later, I hope you still agree. When all is said and done, exercise—despite its manifold benefits—requires overriding deep, natural instincts. So instead of shaming and blaming people who avoid exertion, we should help each other choose to exercise.

Which brings me to a final point. Researching and writing this book has convinced me that a philosophy for how to use one’s body is just as useful as a philosophy for how to live one’s life. All of us get only one chance to enjoy

good life, and we don’t want to die full of regret for having mislived it, and that includes having misused one’s body. By following deep and ancient instincts to avoid the discomfort that comes with physical exertion, we increase the chances we will senesce faster and die younger, and we become more vulnerable to many diseases and chronic, disabling illnesses.

simply. Make exercise necessary and fun. Do mostly cardio, but also some weights. Some is better than none. Keep it up as you age.